This invention relates to gas flow circuit breakers in general, and more particularly to an improved arc quenching arrangement for a blast piston circuit breaker.
Arc quenching, by blowing a gaseous quenching medium at the arc to quench it, is well known in the art with a number of different circuit breaker systems having been developed using this principle.
One of these which is generally referred to as the two-pressure system, stores the quenching medium in a high-pressure tank. During the switching process, the quenching medium flows from the tank through valves, ducts and flow conduits to the quenching chamber, which is at a lower pressure. Thus, in this system, a flow results from a pressure gradient.
In what is referred to as a single-pressure system, a predetermined static pressure is maintained in the switching chamber. During the switching process, the necessary flow of quenching medium is obtained by a compression of part of the volume therein. Typical of this type of system is that known as the blast-piston circuit breakers, in which the motion of a cylinder or a piston is coupled to the motion of the circuit breaker. As the piston moves along with the opening contact, a portion of the volume of the quenching gas is compressed. At a predetermined position of the nozzle or electrode arrangement in the circuit breaker, flow cross sections are opened, permitting the flow of the quenching medium to begin.
In this type of circuit breaker, the quenching gas can interact with the arc. The arc in a gas flow circuit breaker burns between two contacts, one of which is generally a tubular contact. The flow acting against the arc takes place in a nozzle arrangement. The tubular contact can also serve as the nozzle or a separate nozzle preceding the contacts can be provided. It is a general characteristic of all circuit breaker arrangements of this type that the arc must burn through a cavity which can have the shape of a cylinder, a cone or a Laval tube and can be of different length. The arc must burn and the quenching medium must flow through this cavity which forms the nozzle. The arc will impede this flow of quenching medium. As an arc burns through a nozzle, two different zones can be distinguished: an inner, hot zone of lower density and an outer, cold zone of high density. The major portion of the total mass of the quenching means passing through the nozzle will flow through the outer, cold zone. As the arc becomes thicker, the hot zone becomes wider and the cold outer zone becomes correspondingly smaller. As a result, the mass throughput through the nozzle decreases with increasing thickness of the arc. If the arc increases in size, to fill the cross section of the nozzle completely, the mass throughput becomes a minimal.
In a-c circuit breakers, the arc current varies according to the sinuoidal shape of the half-waves of the current. When a short-circuit current is interrupted, an arc almost completely filling the cross section of the nozzle will occur at the time of maximum current. Due to the reduced mass flow, a correspondingly reduced cooling effect is obtained. As a result, the energy given up in the quenching chamber can no longer be carried off by the quenching medium. As a result of this, the pressure in the quenching chamber sharply rises. The pressure increase can lead to a situation where the inflow from the high-pressure portion of the breaker is not only reduced but, in some cases, that the direction of flow is even reversed, causing the hot gas to get into the inlet ducts. As a result, after the current decreases and normal flow direction is again resumed, a quenching medium which is heated and contaiminated with metal vapor from the electrodes will initially flow into the quenching arrangement.
Aside from the effects resulting from the hot quenching gas getting back into the inlet ducts, the pressure increase in the quenching chamber in a blast-piston type breaker can also brake the movement of the piston and with it the contact movement. As the force difference between the force driving the circuit breaker, i.e., driving the blast-piston and the circuit breaker contacts and the opposite force of the compressed gas becomes increasingly smaller, a change in direction of motion is even possible. That is to say, the piston and contacts can be driven in a direction to close rather than open the contacts. Clearly this is undesirable.
Thus, it can be seen that there is a need for an improved arrangement in a circuit breaker of this type which avoids the undesirable effects of the back pressure which can be generated when high currents are being interrupted.